1.Field of the Invention
The present invention relates to a process for manufacturing a cable resistant to external chemical agents.
More particularly, the present invention relates to a process for manufacturing a cable, in particular an electrical cable for low-voltage, medium-voltage or high-voltage power transmission and/or distribution, which comprises at least one conductor, at least one metallic tape coated with at least one adhesive coating layer and at least one coating layer comprising at least one polyamide or a copolymer thereof.
2. Description of the Related Art
Within the scope of the present invention, “low-voltage” generally means a voltage up to 1 kV, “medium-voltage” means a voltage between 1 kV and 35 kV, “high-voltage” means a voltage greater than 35 kV.
Electrical cables generally comprise one or more conductors individually coated with semiconductive and insulating polymeric materials and coated with protective coating layers which are also made of polymeric materials.
It is known that, in cables installed in critical environments such as, for example, oil refineries, oil pools, offshore installations, a major problem is given by the permeability of said polymeric cable coating layers to humidity and, in particular, to aggressive chemicals both of organic type such as, for example, hydrocarbons and solvents, and of inorganic type such as, for example, acids and bases. Penetration of said elements to the interior of the cables compromise their overall lifetime performance both in term of mechanical properties and electrical properties.
A conventional protection against said elements is generally achieved by applying a lead sheath. As a result, lead sheaths are commonly found over insulated wire conductors having, for example, paper/oil insulation, or solid dielectric such as ethylene-propylene rubber insulation, or crosslinked polyethylene insulation. Lead provides flexibility, hermetic sealing capability, and is considered relatively easy to extrude in long lengths. Cables of this type are commercially known, for example, as Solid Type PILC cables from The Okonite Company.
Welded corrugated aluminum (or copper) sheaths are also known to afford cable protection instead of lead sheaths. These aluminum sheaths are relatively light, provide hermetic sealing capability and may serve as a neutral conductor when placed over power cables. Cables of this type are commercially known, for example, as C-L-X® Type cables from The Okonite Company.
However, such sheaths still provide significant weight increase.
In order to avoid the use of both the lead sheaths and the corrugated aluminum sheaths above-mentioned, different solutions have already been proposed in the art.
U.S. Pat. No. 4,125,739 discloses a cable shielding tape comprising a metal strip having a first adhesive layer of polymeric resinous material tightly adhered to at least one side thereof and a bond control layer of polymeric resinous material strippably adhered to the first adhesive layer. Plastic jacketed electric power and communication cables utilizing such shielding tape are also disclosed. Materials which may be used to form the bond control layer include polypropylene, carboxyl modified polypropylene, polyamides, polyethylene therephthalate, fluoro polymers, 1,4-dimethylpentene polymers, ethylene/propylene copolymers, and stereoregular polystyrene. Materials which may be used to form the adhesive layer include polymers or copolymers of ethylene modified by monomers having reactive carboxylic acid groups. The outer plastic jackets of such cables is said to withstand delamination under conditions of normal use but can easily be removed to facilitate grounding and splicing procedures as the adhesive layer remains tightly adhered to the metal strip for protection against corrosion following the removal of the jacket.
U.S. Pat. No. 4,327,248 discloses tubing and electrical cable shields made of a flexible metal tape that has a coating of a copolymer of ethylene with a monomer having a reactive carboxyl group bonded to at least one of its sides and to which coating is bonded an adhesive that is adapted to bond the coating to flexible or semi-rigid non-olefinic polymeric materials. Flexible or semi-rigid non-olefinic polymeric materials which may be used are, for example, polyvinyl chloride or amorphic chlorinated polyethylene, or an elastomeric material such as polyurethane or synthetic rubbers. The adhesive may be selected from polyamide based adhesives.
U.S. Pat. No. 4,675,471 discloses an electrical cable comprising a conductive core and a metallic screen, wherein said metallic screen is coated with a coextruded film comprising a layer of a polymer selected for its properties of high flexural modulus, high tensile strength and high melting point and a layer of adhesive. The polymer layer is a polyamide, a copolyamide, or a copolyester. The adhesive is a copolymer of an olefin and at least one comonomer which is a polymerizable, ethylenically unsaturated carboxylic acid or acid anhydride or derivatives thereof or, alternatively, the adhesive comprises an adhesive blend of the copolymer and a polyolefin.
A cable comprising a sheating system including a longitudinally folded polyethylene coated aluminum tape (PE/AL/PE) is known and is commercialized by Pirelli under the trademark Drylam® sheathing system. During extrusion of the polyethylene jacket onto said aluminum tape, the polyethylene coating present at the overlapping region of said longitudinally folded aluminum tape seals together the overlapping edges providing excellent impermeability to moisture. In addition, the aluminum tape provides protection against electro magnetic interference. During the extrusion of the polyethylene jacket, the polyethylene coating present on the aluminum tape bonds the metallic shield to the polyethylene jacket giving the cable good mechanical properties. Moreover, the polyethylene jacket is highly resistant to inorganic chemicals such as acid and bases. A modified polyamide coating layer is applied with intimate adhesion to the polyethylene jacket. This material is highly resistant to organic chemicals such as hydrocarbons and solvents providing also termite proof and rodent resistant properties in case of non armoured cables.
Applicant has observed that the use of a sheath made of a laminated metal tape coated with an ethylene-based adhesive coating layer and a polyamide coating layer as disclosed, for example, in U.S. Pat. No. 4,675,471 above cited, is not as effective as desired in protecting the cable from the external attacks of both humidity and chemical agents. In particular, Applicant has observed that, when said laminated metal tape is longitudinally folded around an insulated conductor, in particular in the case the edges of said metal tape are overlapped, the risks of penetration of both humidity and chemical agents to the interior of the cable is very high due to the fact that the polyamide present at said overlapping edges does not allow an effective bonding of the overlapping edges. The penetration is due to both a poor bonding of the overlapping edges and a diffusion through the thickness of the adhesive and polyamide coating layers in the overlapping edges region. Moreover, said laminated metal tape has a remarkable thickness which cause an increase of both the cable weight and the cable outer diameter.
The use of the Drylam® sheathing system above disclosed allows to avoid the presence of the polyamide at the overlappings edges of the polyethylene coated aluminum tape thereby improving the bonding at the overlapping edges. However, the presence of a polyethylene coating layer around and in contact with the polyethylene coated aluminum tape is necessary in order to ensure a good adhesion between the coated aluminum tape and the polyamide layer thereby increasing the overall cable diameter.
Therefore, the Applicant has faced the problem of avoiding the use of said additional polyethylene coating layer. The elimination of said polyethylene coating layer would allow to further reduce the cable outer diameter and to manufacture a cable in a more economic way due to both a simplification in the manufacturing process and a cost reduction of the starting materials.
However, the Applicant has observed that, while it is possible to obtain a good adhesion between a metal tape coated with an ethylene-based adhesive coating layer and a polyamide coating layer by means of a calendering process, the same adhesion was not obtained by means of an extrusion process. In particular, the Applicant has observed that the extrusion of a polyamide coating layer onto a longitudinally folded metal tape coated with an ethylene-based adhesive coating layer did not allow an effective coupling between the coated metal tape and the polyamide coating layer.